Coating systems that are deposited onto a substrate and cured, such as “color-plus-clear” and “monocoat” coating systems, can be subject to damage from the environment. For example, corrosion of a coated metallic substrate can occur as the substrate is exposed to oxygen and water present in the atmosphere. As a result, a “primer” coating layer is often used to protect the substrate from corrosion. The primer layer is often applied directly to a bare or pretreated metallic substrate. In some cases, particularly where the primer layer is to be applied over a bare metallic substrate, the primer layer is deposited from a composition that includes a material, such as an acid, such as phosphoric acid, which enhances the adhesion of the primer layer to the substrate. Such primers are sometimes known as “etch primers”.
As indicated, in some cases metallic substrates are “pretreated” before a primer coating layer is applied (if such a primer coating is used). Such “pretreatments” often involve the application of a phosphate conversion coating, followed by a rinse, prior to the application of a protective or decorative coating. The pretreatment often acts to passivate the metal substrate and promotes corrosion resistance.
Historically, corrosion resistant “primer” coatings and metal pretreatments have utilized chromium compounds and/or other heavy metals, such as lead, to achieve a desired level of corrosion resistance and adhesion to subsequently applied coatings. For example, metal pretreatments often utilize phosphate conversion coating compositions that contain heavy metals, such as nickel, and post-rinses that contain chrome. In addition, the compositions used to produce a corrosion resistant “primer” coating often contain chromium compounds. An example of such a primer composition is disclosed in U.S. Pat. No. 4,069,187. The use of chromium and/or other heavy metals, however, results in the production of waste streams that pose environmental concerns and disposal issues.
More recently, efforts have been made to reduce or eliminate the use of chromium and/or other heavy metals. As a result, coating compositions have been developed that contain other materials added to inhibit corrosion. These materials have included, for example, zinc phosphate, iron phosphate, zinc molybdate, and calcium molybdate particles, among others, and typically comprise particles having a particle size of approximately a micron or larger. The corrosion resistance capability of such compositions, however, has been inferior to their chrome containing counterparts.
As a result, it would be desirable to provide coating compositions that are substantially free of chromium and/or other heavy metals, wherein the compositions can, in at least some cases, exhibit corrosion resistance properties superior to a similar non-chrome containing composition. In addition, it would be desirable to provide methods for treating metal substrates, including bare metal substrates, to improve the corrosion resistance of such substrates, wherein the method does not involve the use of chromium and/or other heavy metals.